This invention relates to an injection moulded plastic article and is particularly directed to a novel rib lattice structure applied to substantially planar surfaces of injection moulded plastic articles.
In the field of manufacture by injection moulding and the merchandising of plastic articles, particularly thin-walled lid-like articles such as the lids for containers for food and beverages and articles such as trays and dishes which are all produced in mass quantities, minimization of the plastic content of the article and maximization of the allowable speed of moulding production of the article are both of high economic desirability.
Of significance in connection with this present application is the general nature of my invention disclosed in U.S. Pat. No. 3,679,119 which achieved a substantial improvement over the prior art in these two areas of economy in respect to thin-walled cup-like articles. The principal element in the aforesaid invention is a cross-lattice of intersecting ribs for application to the side wall or walls of the article dividing the wall surfaces bearing the network of ribs into a plurality of discrete rhomboidal web portions entirely bounded by said ribs, the said ribs being directed at an acute angle to the theoretical line of shortest distance in the walls between the point of injection and the moulding terminus of the article, said ribs commencing at the bottom of the side wall and extending toward the upper terminus of the side wall, each set of criss-crossing ribs respectively being substantially parallel to each other 360 degrees around the vertical axis of the cup-like article.
The rib lattice of my prior invention allows the reduction by as much as 35% of the material content of a cup-like article by permitting the moulding of extreme thinnesses in the rhomboidal rib-enclosed webs without the reduction of the structural strength of the side-walls below acceptable limits. Also it has been found that my prior invention allows increases ranging from 15% to as high as 35% over the production moulding speed for articles of conventional smooth-walled design because of the faster cooling of the reduced side-wall thicknesses.
The rib lattice structure of my prior invention, being designed for application to the side walls of cup-like articles, achieved the greatest measure of the aforementioned economies when applied to articles in which the side wall or walls comprised the major component of the article. As that invention was put into service it became apparent that there was an urgent need to achieve these same economies in a different class of plastic articles, namely lid-like articles such as lids for containers, dish-like or tray-like articles and the bottom wall elements of wide-bottomed containers in which a major portion of the total article comprised a substantially planar wall with the point of injection for the article located centrally in the planar wall, hereinafter referred to as "planar articles". This need for economy derived from the long-accepted design requirement that such planar walls must be of equal or greater thickness than the thickest section of the side wall or rim structure surrounding the planar wall in order to ensure satisfactory filling of the mould during moulding, and an almost invariable result has been that the thicknesses so required have been far in excess of the functional strength needs of the said planar wall.
Several serious problems had to be considered in contriving a rib lattice structure which would achieve any measure of economy in such planar walls comparable to what had been gained in respect to the side walls of cup-like articles.
First, it was evident that the cross-lattice rib structure of my prior invention could not be effectively utilized to serve the planar wall of a lid-like article since the structure defined and illustrated in the aforesaid disclosure obviously required each of the two sets of criss-crossing ribs respectively to remain substantially parallel to each other 360 degrees around the vertical axis passing through the point of injection of the article. The reason for this parallelity requirement is that the main economy-yielding elements of the side wall rib lattice are the discrete thin rhomboidal webs resulting from application of the rib lattice structure and that these rhomboidal webs must be of substantially the same optimum area and shape over the entire area served by the rib lattice. This optimum must be determined individually for each article designed taking into account such variables such as the size of the article and the distance the molten plastic must flow to fill its mould, the melt viscosity of the particular plastic to be used, the injection pressures available on the moulding machine to be used and the functional strength required of the article. No geometrically practical configuration of criss-crossing ribs in which each set of ribs respectively remained parallel to each other 360 degrees around the centre of a planar wall could be found capable of producing the rhomboidal web structures which are essential to the side wall economies of my prior invention. And, abandoning the parallelity requirement, to project the angular ribs of the side wall rib lattice towards the central point of injection proved of no avail because the ribs converged at such a rate that the disparity in shape and area of the rhomboidal webs nearest the point of injection as compared with those nearest the perimeter was unacceptably large.
It should be explained herewith that the reason for the rhomboidal shape of the web enclosure is that I have found that the angle of departure of any ribs from the direct line of flow of plastic during mould filling must be confined to the range of 10 to 45 degrees so that the diversion resistance to the plastic flow is minimized and yet the criss-crossing ribs intersect to provide easy multi-directional filling of the thin webs.
Another serious problem, the problem of post-mould warpage, seemed to forbid the imposition of any kind of rib lattice on the flat planar wall of a lid-like article for it has been a long recognized experience in practice of the art that any unevenesses in the thickness of a planar wall, such as the presence of ribs on the wall, resulted in variations in the rates of cooling and solidification in various local areas and consequently, in variations of linear shrinkage, which, in the case of many common plastics, is inversely proportional to the rate of cooling. The danger to be avoided was the locking in of varying local shrinkage stresses with unacceptable warpage of the article occuring shortly after release from the mould.
Straight radial ribs running along the line of plastic flow from the point of injection to the perimeter of the planar wall were known in the prior art, as shown in U.S. Pat. No. 3,944,124 to Hexel and in Canadian Pat. No. 940,068 to Fennema, but it was a forbidding prospect that the complex pattern of a criss-cross rib lattice could be imposed on a planar wall without post-mould warpage.
Still another problem was that the geometric design specification of a rib lattice suited to integration with a substantially planar wall such as in a lid-like article had to be readily adaptable to provide whatever shape and size of rhomboidal web structure determined to be the above-mentioned optimum for each particular application.
And there was the all-important requirement that, while providing a net saving in material content, the rib lattice structure must provide that same ease of plastic flow across the planar wall as the conventional thick wall of the prior art.
The present invention provides a rib lattice structure which, when applied to a substantially planar wall of an article in which the point of injection is centrally located, overcomes the aforesaid problems. The structure provides a plurality of rhombodial webs all of the same area and shape over the area served by the rib lattice, and is of a defined geometric structure readily adaptable to serve all sizes and shapes of planar walls. Its effect is to provide such planar walls of planar articles with equal or greater economies as provided by the rib lattice of distinctly different design does for the side walls of cup-like articles.
A variety of theories can be offered to explain how the novel design of rib lattice of the present invention overcomes the formidable problems cited above and achieves substantial economies in planar articles.
First, the passageways provided in the closed mould to allow formation of the rib network, being of relatively large cross-section as compared to the moulding spaces for formation of the webs, serve as an interlocking network of conduits for the inrushing molten plastic during the moulding of the article and permit the filling of thinner basic walls than heretofore practical in planar walls since each discreet web is completely surrounded by ribs and this is "flash" filled from multiple directions from the ever-freshened supply of hot plastic flowing in the rib "conduits". It has been found that webs as thin as 0.025 centimeters can be filled successfully with the aid of this rib lattice. A substantial material economy in the plastic content of a planar wall ranging from 20% to 40% over that of a conventional smooth wall results from the fact that the very thin web areas comprise between 25% and 50% of the total planar wall and the total mass of the ribs constitute a very small fraction of the total mass of the wall in various applications.
The network of intersecting ribs, in addition to aiding the filling of very thin web areas, serves to conduct the molten plastic just as readily to the moulding terminus of the article as in the case of a conventional smooth planar wall, the ribs being of sufficient cross-section to retard the cooling of the plastic therein. Also these interlocked ribs effectively enhance the rigidity and strength of the said planar wall, off-setting any loss of strength from the material reduction.
Still another contribution of my present rib lattice structure is to the strength-to-weight ratio of a thin planar wall for there appears to be a notable reduction in the residual internal stress from unsatisfied shrinkage in the finished article. The explanation of this benefit would appear to be as follows: In the case of moulding of a conventionally designed thin-walled article in which the walls are smooth and of equal thickness at all points, the plastic solidifies in all areas at the same instant while the article is still in the rigid confines of the tightly closed mould and a complex pattern of internal stress arising from shrinkage of the plastic during its change to the solid state is locked into the wall, having no direction in which to relieve itself. However, a different situation prevails in a wall bearing a lattice of intersecting ribs when the plastic freezes in the mould. First the plastic in the thin webs freezes and much of the shrinkage stress which would normally remain in the webs relieves into the close-by ribs which are still molten by reason of their greater cross-section mass. When the rib runs freeze they in turn relieve stress into the adjacent rib intersections which are the last points to solidify, again because of their still greater cross-section mass.
Not only does reduction of internal stress improve the impact resistance of any thin walled plastic article, but this effect may explain why we escape the problem of post-mould warpage in applying the rib lattice structure to the planar wall of a planar article. Another reason may be that the total rib lattice of the invention is a balanced isometric configuration and that any local shrinkage stresses remaining after moulding cancel each other out.
A further benefit of the present invention has been found to be a tendency of the angular ribs to induce the plastic to flow in a multi-oriented pattern during moulding, resulting in a multi-oriented grain in the plastic of the planar wall, minimizing the susceptability of the wall to splitting when stressed.
In the production by injection moulding of such divers articles as large pails, serving trays, and the like vessels and containers with large planar wall areas, it is sometimes the practice to provide the mould with two or more points of injection. Thus, the present invention seeks to provide a rib lattice structure adaptable to interfacing with two or more duplications of the same structure in configurations which will achieve the same advantages as when only one point of injection is utilized, it being understood that the term "planar articles" used herein is intended to encompass the aforementioned articles.